In your other thread you said you want a UK EASA CPL. No, they will not put level 6 or 5 or 4 on it in any other language, because there is no requirement to do so. It does not stop you speaking French though for those fields where you need to. An RT Licence can be issued in any language of the Union (ITU). It does not seem to be presenting other French pilots a problem. I have signed up several for Level 6 English to put on a UK issued EASA licence. They all wanted a UK EASA licence in preference to a French one. Nobody can insist you have a language on your licence for which there is no EU or ICAO requirement. An EASA licence is valid in all States.
they informed me that national authorities are not obliged to recognise national qualifications from other JAR members,
This is true which is why I suggested you follow the conversion route in the State that issued the ratings. Once they are on a EASA licence, then they are acceptable.